The present invention relates to a method of sieving and more particularly to a method of sieving for use in determining the fineness of a granular material such as, for example, fly ash.
It is often necessary to determine the fineness of a granular material such as, for example, fly ash, and various tests have been proposed for determining the fineness of a finely divided material of this type. One such test is described in Australian Standard 1129, and another such test is described in ASTM C113. Both these standard tests measure the fineness at given points in the particle size grading by measuring the proportion of the material having particles with a diameter of greater than 150 .mu.m and also the proportion of the material having particles with a diameter of greater than 45 .mu.m.
By way of comparison, the British Standard method for determining the fineness involves the determination of the specific surface area, which does not directly give a measurement of particle size. The British Standard test is a difficult and laborious test to perform, and it has been found that a time of at least 45 minutes is necessary to complete the test. In this country, as an alternative to determining the specific surface area, wet sieving of fine materials has been adopted as a more reproductable test method, but a time of 25 minutes is needed to complete a wet sieving test, the duration of this period of time being mainly due to the time necessary subsequently to dry the materials before weighing. It has been proposed to utilise a rapid drying method, to reduce the time necessary to conduct this test, but it has been found that this rapid drying method eventually results in damage being done to the sieve.
It has been proposed to utilise an apparatus for sieving dry material in which a stream of air passes upwardly through the sieve into the area containing the material to be sieved thus stirring or agitating the material, a partial vacuum source subsequently drawing air and particles of small size through the sieve. Preferably proposed apparatus of this type is extremely expensive.